A wide variety of information systems applications exist that require a high density of interconnections among device or system nodes, or a high density of rapidly accessible memory, or both. These applications include, for example, neural networks, telecommunications switching systems, digital computing, and information (including signal) processing. In many such applications, key requirements on the chosen interconnection technology include low insertion losses, high interchannel isolation (freedom from interchannel crosstalk), a high degree of potential fan-in and fan-out at each node, weighted interconnection channels, and high capacity. Comparable requirements exist for the chosen memory technology, including low latency (rapid information access), parallel information retrieval, low effective bit error rates (high signal-to-noise ratio), high density information storage, and input/output compatibility with the remainder of the system.
In order to satisfy these many and varied requirements, multiplexed volume holographic optical elements provide an attractive alternative to electronic implementations of high capacity interconnection and memory elements. In fact, the very nature of a volume holographic optical element tends to blur the distinction between a pure interconnection network on the one hand, and a pure memory subsystem on the other, as it is in many ways simultaneously well-suited to both roles. Even so, previous methods for recording information or interconnection patterns in highly multiplexed volume holographic optical elements, and for reading them out, have not proven satisfactory in terms of throughput, crosstalk, and capacity. Furthermore, they have not proven to be manufacturable, due to the fact that information from a master volume holographic optical element could not previously be efficiently transferred to or duplicated in another such element.
In forming multiplexed volume holograms, one of three approaches is typically taken: (1) sequential, which involves several temporally-sequenced (and hence incoherent) exposures of the individual components of the hologram, done by rotating or translating the hologram (or the source beam, reference beam, or object beam); (2) simultaneous and fully coherent, which involves the use of two or more mutually coherent beams, each encoded with information and serving as a reference beam for the other(s); and (3) some combination of sequential and simultaneous fully coherent.
The first approach has the major disadvantage that temporal sequencing is time-consumptive, which can be of considerable importance in applications envisioned herein, for which the number of independent interconnections that must be recorded is extremely large. Also, in many holographic recording materials, sequential exposures tend to erase previously recorded information, leading to the necessity of incorporating unwieldy programmed recording sequences in order to result in the storage of a predetermined set of interconnections.
The second approach is designed to circumvent the above sequencing difficulties, but suffers instead from the coherent recording of unwanted interference patterns (holograms) that give rise to deleterious crosstalk among the various (supposedly independent) reconstructions, as described in more detail below.
The third approach is subject both to sequential recording time delays and the necessity for programmed recording schedules, as well as to the generation of undesirable crosstalk. As such, none of the previously employed multiplexed recording techniques allows for the generation of three-dimensional, truly independent interconnections between two or more two-dimensional planar arrays within the context of a temporally efficient recording scheme.
In all of the prior art approaches to the holographic recording of a multiplexed interconnection, two primary forms of interchannel crosstalk are encountered to a greater or lesser extent. Coherent recording crosstalk arises from the simultaneous use of multiple object and reference beams, all mutually coherent with each other. The mutual coherence causes additional interconnections to be formed other than those desired. Reconstruction with independently valued inputs results in the generation of output beams that cross-couple through the undesired interconnection pathways, which compromises the independence of the desired interconnection channels.
A second, unrelated form of crosstalk arises due to beam degeneracy, which occurs whenever a single object beam is used with a set of reference beams to record a fan-in interconnection to a single output node (e.g., neuron unit in the case of the photonic implementation of neural networks). (Fan-in is the connection of multiple interconnection lines to a common output node.) This latter form of crosstalk is present even when the set of object beams is recorded sequentially.
Of at least equally serious consequence is the optical throughput loss that results from interconnection fan-in so constructed as to exhibit beam degeneracy. In many well-documented cases, this loss is severe, resulting in at least an (N-1)/N loss (or, equivalently, a 1/N throughput efficiency) for the case of an N-input, N-output interconnection system, as reported by J. W. Goodman, Optica Acta, Vol. 32, pages 1489-1496 (1985). This is a truly daunting loss factor for interconnection systems such as those envisioned for neural networks, which may both require and be capable of 10.sup.5 to 10.sup.6 inputs and outputs.
In certain types of photorefractive materials, an additional throughput loss can arise from the incoherent superposition of several gratings within the same volume of the holographic optical element, due to the reduction in the effective modulation depth of the recorded holographic fringes. This effect occurs primarily in photorefractive crystals that generate an index of refraction or absorption change in response to local gradients in the intensity distribution, but would not be expected to occur in linear photorefractive materials that generate an index of refraction or absorption change in direct proportion to the magnitude of the local intensity distribution. In a number of cases, this effect can also result in at least an (N-1)/N loss for the case of an N-input, N-output interconnection system, as reported by P. Asthana, "Volume Holographic Techniques for Highly Multiplexed Interconnection Applications", Ph.D. Dissertation, University of Southern California (1991), available from University Microfilms, Ann Arbor, Mich.
In the prior art, few attempts have been made to address the extremely important technological problem of duplicating the contents of a fully recorded, heavily multiplexed volume holographic optical element or interconnection device, particularly in the case of neural network interconnections. For example, to the inventors' knowledge, there is no known prior technique for rapid copying of a volume hologram that is angularly multiplexed in two dimensions, other than that described in the parent application of the present application.
In the case of neural network interconnections, the training and/or learning sequences may be quite involved; in some cases, the training and/or learning sequences may result in a unique interconnection, and the exact learning sequence may not be reproducible in and of itself at all. In such cases, it is desirable to replicate the contents of the interconnection medium in such a manner that a fully functional copy is produced, as characterized by a complete operational set of interconnections indistinguishable from those implemented by the master. The method of replication must not demand an extremely lengthy recording sequence, must not be inefficient in its utilization of the programmed recording schedule and/or the total optical energy available for reproduction purposes, and must not induce additional optical throughput loss or interchannel crosstalk beyond that already incorporated in the master.
In the parent application of the present application, apparatus for the incoherent/coherent multiplexed holographic recording of photonic interconnections and holographic optical elements is described. Specifically, apparatus for providing multiplexed volume holographic recording and readout comprises:
(a) means for providing an array of coherent light sources that are mutually incoherent; PA1 (b) means for simultaneously forming an object beam and a reference beam from each coherent light source, thereby forming a set of multiplexed object beams and a separate set of multiplexed reference beams; PA1 (c) means for either independently modulating each object beam, or spatially modulating a set of object beams so that all object beams are identically modulated; PA1 (d) means for either independently modulating each reference beam, or spatially modulating a set of reference beams so that all reference beams are identically modulated; PA1 (e) a holographic medium capable of simultaneously recording therein a holographic interference pattern produced by at least a portion of the set of all modulated multiplexed object beams and of the set of all modulated multiplexed reference beams pairwise, with all such pairs being mutually incoherent with respect to one another; and PA1 (f) means for directing at least a portion of the set of modulated object beams and of the set of modulated reference beams onto the holographic medium and for interfering the portion of the modulated object beams and of the set of modulated reference beams, pairwise, inside the holographic medium. PA1 (a) means for providing an array of coherent light sources that are mutually incoherent; PA1 (b) means for forming two reference beams from each coherent light source, thereby forming two sets of multiplexed reference beams, each set at a different location; PA1 (c) means for directing the first set of reference beams onto the original multiplexed volume hologram to thereby form a set of output beams; PA1 (d) means for directing the second set of reference beams onto a secondary holographic recording medium; PA1 (e) means for directing the set of output beams from the original multiplexed volume hologram onto the secondary holographic recording medium, with path lengths sufficiently identical to the reference beam path lengths to permit coherent interference, pairwise, between the output beams and the second set of reference beams, inside the secondary holographic recording medium; and PA1 (f) means for simultaneously recording in the secondary holographic medium a holographic interference pattern produced by the set of output beams and the second set of reference beams, thereby forming the substantially identical multiplexed volume hologram. PA1 (a) the reconstructed beams that emanate from the holographic medium are at least partially angularly multiplexed; PA1 (b) a spatial array of pixels is encoded onto each reconstructed beam, as an image at some plane in space; and PA1 (c) the images of said spatial arrays of pixels from the reconstructed beams can be made to be substantially coincident in space. PA1 (a) means for directing at least a portion of each beam of a set of mutually incoherent reference beams onto the primary volume holographic optical element, thereby producing a set of output beams; PA1 (b) means for directing at least a portion of each beam of the set of mutually incoherent reference beams onto the secondary holographic medium; PA1 (c) means for directing the set of output beams from the primary volume holographic optical element onto the secondary holographic medium, with beam path geometries appropriate to permit coherent interference between each output beam and each corresponding reference beam, inside the secondary holographic medium; and PA1 (d) means for recording in the secondary holographic medium a holographic interference pattern produced by the set of output beams and the portion of each beam of the set of mutually incoherent reference beams.
Although the primary mode of multiplexing is angular, spatial and/or wavelength multiplexing may also be incorporated.
The architecture and apparatus described in the parent application significantly reduce coherent recording cross-talk and beam degeneracy crosstalk, and permit simultaneous network initiation, simultaneous weight updates, and incoherent summing at each output node without significant fan-in loss.
Further in accordance with the parent application of the present application, the above apparatus is provided with means for controllably blocking the set of object beams such that at least a portion of the set of reference beams (either modulated or unmodulated) reconstruct a stored holographic interference pattern. In one embodiment, the reconstructed pattern is angularly multiplexed and detected in such a manner as to produce an incoherent summation on a pixel-by-pixel basis of the reconstructed set of object beams. In this manner, multiplexed volume holographic recording and readout are provided.
Specific implementations to neural networks, telecommunication interconnections (e.g., local area networks and long distance switching), interconnections for digital computing, and multiplexed holographic optical elements are provided in the parent application.
In addition, apparatus for copying a multiplexed volume hologram is provided in the parent application. The apparatus comprises:
Portions of the above-described apparatus also possess unique properties and give rise to useful functions. It is to these unique portions, or elements, that the present application is directed.
In addition, specific implementations are given that utilize subhologram formation to avoid throughput losses due to incoherent superposition effects, that provide for various modes of information transfer from a master hologram to a copy, that address application areas in optical memory and optical signal processing, and that exploit the double angular multiplexing features of the apparatus described in the parent application.